1. Field of the Invention
The present invention relates to an optical recording medium with a colored image which is stable against heat and visible light.
2. Description of the Prior Art
Photochromic compounds are known for forming colored images upon irradiation with ultraviolet rays. A typical example of a photochromic organic compound which is currently being used is a spiropyran compound. When a spiropyran compound is used as a photochromic material, the spiropyran compound is dispersed or dissolved in a polymer serving as a binder resin and is formed into a film or coated on a base. Such a conventional photochromic material becomes colored upon irradiation with ultraviolet rays and then returns to its initial colorless state upon heating or irradiation with visible light. Since a photochromic material containing a spiropyran compound has such interesting properties, a variety of applications for different recording materials, copying materials, and display materials have been attempted with such compounds.
However, the conventional photochromic compound is not thermally stable in a colored state. The colored compound gradually fades even at room temperature and its colored state cannot be maintained for a long period of time. Even when the photochromic compound is exposed to visible light, its colored state is lost. Thus, when the photochromic image must be kept for a long period of time, it must be shielded from visible light. When an optical recording medium using a conventional photochromic material is utilized for enlarging and projecting a photochromic image, the image is discolored by the light from the light source within a short period of time. For this reason, current applications of optical recording media using conventional spiropyran compounds are limited.